The present invention is directed to an improvement in ball clutch mechanisms for restraining a pin from longitudinal movement.
Ball clutch mechanisms are commonly used with pins for attaching items together. One such use is in the electronic article surveillance field wherein tags that can be sensed electronically include a ball clutch mechanism for enabling the tag to be attached to an article of merchandise by inserting a pin through the article and into the ball clutch mechanism of the tag. Ball clutch mechanisms for such a tag are described in U.S. Pat. Nos. 4,221,025 to Martens and Vanderbult; 3,911,534 to Martens and Vanderbult and 3,858,280 to Martens.
The ball clutch mechanism described in U.S. Pat. No. 4,221,025 is presently in commerical use. Basically, such ball clutch mechanism includes an anvil having an axial bore for axially receiving the pin; a generally radially symmetrical cup having a confining end with a small axial opening therein for admitting the pin, a tapered interior wall and a predominantly open end covering the anvil and axially aligned with the anvil for axially receiving the pin that passes therethrough and into the bore of the anvil, wherein the anvil is longitudinally movable along its bore axis with respect to the cup; a spring for forcing the anvil toward the closed end of the cup; and a set of uniformly dimensioned balls in the cup, engaging the anvil and forced by the anvil toward the confining end of the cup to clutch the pin when the anvil is forced toward the confining end of the cup by the spring. The interior wall of the cup is dimensioned and tapered with respect to the balls such that when the anvil is forced toward the confining end of the cup, the balls are wedged between the tapered interior wall of the cup and the pin to apply radial pressure against the pin to clutch the pin for restraining the pin from longitudinal movement. The pin is released from the ball clutch mechanism through use of a detaching tool described in U.S. Pat. No. 3,911,534. Such tool includes an electromagnetic for drawing the anvil (which is made of magnetic material) of the ball clutch mechanism against the force of the spring to thereby release the balls from the tapered wall at the confining end of the cup and remove the radial pressure applied to the pin.
To prevent inadvertent release of the pin it is necessary that the balls maintain symmetrical radial pressure on the pin. In an attempt to so provide such symmetrical radial pressure on the pin, the ball clutch mechanism described in U.S. Pat. No. 4,221,025 includes rigid mechanical apparatus for uniformly spacing the balls and for precluding annular movement of the balls when the pin is inserted into the ball clutch mechanism. In alternative embodiments described therein such apparatus consists of (1) radially extending open channels on the surface of the anvil that engages the balls, through which channels the balls can freely pass for engaging the interior side wall of the cup; (2) a plurality of longitudinally radially inwardly directed and uniformly spaced projections formed on the interior wall of the cup; (3) uniformly spaced flanges extending from the periphery of a washer positioned between the anvil and the balls; and (4) uniformly spaced flanges extending from the periphery of the anvil and defining channels therebetween through which the balls travel for engagement with the interior wall of the cup. Such rigid mechanical apparatus achieve the desired result only when they are constructed in accordance with extremely tight manufacturing tolerances so as to be precisely symmetrical. In practice the commercial embodiments are not always symmetrical; and as a result the pressure applied to the pin by the balls is not symmetrical and the pin releases from the ball clutch mechanism inadvertently.